Severe allergic reactions to vaccines are rare and difficult to predict. An allergic reaction may be defined as an idiosyncratic reaction that is caused by an immunologic mechanism.

The World Allergy Organization (WAO) has recommended categorizing immunologic reactions to drugs (including vaccines) based upon the timing of the appearance of symptoms [1]. This system defines two general types of reactions: immediate and delayed. This approach is intended to distinguish immunoglobulin E (IgE)-mediated (type I immunologic reactions), which account for many immediate reactions, from other types, because these reactions carry the risk of life-threatening anaphylaxis if the patient is reexposed (table 1).

●Immediate reactions begin within one hour of administration and may begin within minutes. IgE-mediated reactions are most likely to present within this time period.

●Delayed reactions appear several hours to days after administration. These reactions may be caused by several different mechanisms, but they are rarely IgE-mediated.

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